1) Field of the Invention
The present invention relates to a coating rod for paper or cardboard manufacturing machines.
2) Description of the Prior Art
Coating rods are used in the paper industry for conveying a coating material (or xe2x80x9csaucexe2x80x9d) on to cardboard or paper in order to produce a coated cardboard or coated paper. The coating rod is constituted by a metal bar having a diameter generally between 9 and 20 mm and a length generally between 2 m and 10 m. The coating rod is generally held in some sort of support, such as a polymer sleeve, which is also referred to as a bed, in which it can turn. The coating rod, when turning, constitutes also a scraper which removes the excess coating material or xe2x80x9csaucexe2x80x9d from the paper, the measuring out of the deposited coating material or the obtaining of a desired thickness of material on the paper being obtained by varying the pressure of application of the coating rod on the paper and/or by varying the speed of rotation.
The industrial limits of such a method led to designing another type of coating rods which make it possible to increase the amount of coating material deposited on the paper. The improvement consisted in winding a metal wire around the coating rod. By using metal wires of different diameters, the free volume between two consecutive turns of wire can be varied, which makes it possible to obtain a larger or smaller reservoir measuring out coating material. Subsequently, it has been preferred to engrave grooves or flutes in the coating rod, thereby obtaining a profiled coating rod, said flutes or grooves having the same function as the wound metal wires.
Coated paper is a paper with high added value and in general of very high quality, the paper being obtained by depositing two or three coats of coating material on each side. To that end, the machines comprise four, and even more, coating heads. With the speed of paper production continually increasing, transfer rollers are being used more and more in the paper manufacturing machines, the coating rod still being used for depositing a given amount of coating material on the paper. On these machines, the coating rod is used for a xe2x80x9cpre-coating on size pressxe2x80x9d. It follows that an intensive use of the coating rods in the manufacturing machines leads to rapid wear of said rods, said wear being accelerated when a pigmented coating material is used because of the high abrasion of the pigments on the coating rods. The consequence of this rapid wear is that it is necessary to change the coating rods of each machine every second day. As at present the machines each comprise eight coating rods, the impact of coating rods on paper manufacture costs can be assessed without difficulty.
In order to attempt to reduce the wear of the coating rods, a chrome surfacing layer has been produced on said coating rods. But the coating rods produced in this way do not give complete satisfaction even though appreciable advances have been made.
The aim of the present invention is to propose a smooth or profiled coating rod whose lifetime has been considerably increased, preferably by at least 50%.
One object of the present invention is a coating rod which is coated with a surfacing layer constituted by at least one ceramic, said surfacing layer having an angle of wetting or wettability between 30xc2x0 and 90xc2x0 and a porosity less than 5%, the pore size not exceeding 5 xcexcm.
Because of the different surface tensions which exist between the coating rod and the coating material on one hand, and the transfer roller with which the rod cooperates and the coating material on the other hand, the behaviour of the coating material can vary from one situation to another. It has been found that it is necessary for the surface tension between the transfer roller and the coating material to be greater than the surface tension existing between said coating material and the coating rod, if it is wished that all or almost all of the coating material contained, for example, in the hollows of the flutes be transferred. If the wettability of the surfacing layer of the coating rod is not sufficient, then dry friction occurs between the surfacing layer and the coating material. But, if the wettability of the surfacing layer of the coating rod is too great, then at least some of the coating material remains on the surfacing layer, for example in the bottom of the flutes. In the latter case, the flutes do not fully perform their measuring out function since a certain amount of coating material remains trapped in the bottom of the flutes.
Therefore, according to one feature of the present invention, the angle of wetting or wettability of the surfacing layer is between 30xc2x0 and 90xc2x0 and, preferably, between 60xc2x0 and 85xc2x0.
Therefore, according to another feature of the invention, the porosity of the surfacing layer is less than 5% and the pore size does not exceed 5 xcexcm, for avoiding excessively high coating material retentivity.